Hypothermia, which is the condition of lowered normal body temperature in a warm blooded animal, has been used therapeutically for the treatment of human brain injury, spinal cord injury, and spinal cord ischemia for many years. Therapeutic hypothermia is used as a neural protectant in situations where there is central nervous system ischemia, trauma, or hypoxia. Such situations can occur from stroke, global ischemia, accidental trauma, or may be induced during surgery. The induction of therapeutic hypothermia followed by rewarming is now an integral part of modem surgery. In fact, many surgical procedures currently performed are successfully accomplished with the aid of hypothermia. Hypothermia as a medical therapy is recognized as a method of preserving brain tissue in the setting of focal and global ischemia.
Normal core temperature for humans is in the range of 36-38° C. Medical and surgical hypothermic therapy is considered “mild hypothermia” when the core temperature is cooled to approximately 32-35° C., and “deep hypothermia” when the core temperature is reduced below 32° C. Hypothermia has been shown in animal models to reduce the permeability of the blood brain barrier. It also causes a reduction in metabolic activity, along with decreased cerebral demand for glucose and oxygen. Other effects of hypothermia on the brain and spinal cord include decreased rates of high-energy phosphate depletion, decreased lactate accumulation, reduced biosynthesis, and decreased release and uptake of some neurotransmitters. Hypothermia also lowers intracranial pressure and can inhibit brain edema. Individual organ hypothermia, such as that used on transplanted organs, is also used to preserve organs other than the central nervous system.
Known techniques for inducing hypothermia involve both surface cooling and internal cooling, sometimes in conjunction with drugs that disable homeostatic responses. For example, some present methods for inducing hypothermia by surface cooling include externally applied cold packs, whole body ice, ice blankets, immersion in ice water, and chilled forced air refrigeration. Some internal cooling techniques include methods such as infusion of cold saline into veins, arteries, and/or into the peritoneum, and direct extracorporeal cooling.
Surface techniques for achieving and maintaining central nervous system hypothermia are generally unsatisfactory for a number of reasons. First, the organ that is generally the focus of cooling, the central nervous system, is actually cooled last with such techniques. Also, the time to cool the central nervous system can be in the order of four hours or greater. Further, the inertia produced by a steady drop in temperature can produce an “overshoot” condition in which organ damage from excessive hypothermia can occur. Additionally, surface cooling, especially for deep hypothermia, often requires supplemental internal cooling and pharmacological assistance. Cooling by cold blankets and dressings is also often uncomfortable and induces shivering in the patient. Furthermore, there is a risk of body surface injury, and the need to anesthetize for deep hypothermia is often required. Similar drawbacks occur in warming or re-warming the body, such as re-warming a patient after hypothermia.
Internal cooling, on the other hand, is an invasive process which is associated with other problems, including infection, thrombosis, arrhythmia, and cardiac ischemia. Another problem is that the entire body is unnecessarily cooled in situations where only the central nervous system should be cooled. Because there are such problems associated with current methods of inducing hypothermia, a non-invasive cooling system and technique that is focused on the target organ, most notably the central nervous system, would be desirable. It is also desirable to have a non-invasive system for warming the body selectively.
Therefore, it is an object of the invention to provide targeted central nervous system cooling and warming. It is also an objective of the invention to provide a hypothermia/warming system and technique that minimizes or avoids an overshoot cooling or warming of the body. It is another objective of the invention to cool/warm the central nervous system in a non-invasive manner. It is further an objective of the invention to develop a hypothermia/warming system and technique, which can work concurrently and synergistically with existing cooling/warming systems.